Power Management Unit Pmu
Power Management Unit (PMU) Integration is the on-chip subsystem responsible for generating, regulating, sequencing, and monitoring all internal supply voltages required by a complex SoC — ensuring each power domain receives clean, stable power while enabling dynamic power management and safe startup/shutdown sequences.
PMU Architecture Components:
- Voltage Regulators: integrated LDOs (low-dropout regulators) provide clean local supplies from external rails — typical SoC includes 5-20 LDO instances for analog, digital, I/O, and memory domains with dropout voltages of 100-200 mV
- Switched-Capacitor Converters: charge-pump based DC-DC converters achieve higher efficiency (80-90%) than LDOs for large voltage step-down ratios — 2:1 and 3:1 converters common for generating core voltages from battery
- Buck Converter Controllers: on-chip digital controllers drive external power FETs and inductors for high-current domains (>500 mA) — compensator design uses Type-III or digital PID with programmable coefficients
- Bandgap Reference: CTAT (complementary to absolute temperature) and PTAT currents combined to produce temperature-independent voltage reference (typically 1.2V ± 0.5%) — serves as accuracy anchor for all regulators
Power Sequencing and Control:
- Startup Sequence: PMU powers domains in defined order — analog references first, then always-on domain, IO domain, core logic, and finally accelerators — violating sequence can cause latch-up or undefined logic states
- Shutdown Sequence: reverse order with controlled discharge of decoupling capacitors — retention registers saved before power removal to enable fast wake-up
- Power State Machine: finite state machine manages transitions between active, idle, sleep, deep-sleep, and hibernate states — each state defines which domains are powered, at what voltage, and with what clock
- Ramp Rate Control: soft-start circuits limit inrush current during power-up by gradually increasing output voltage — prevents supply droop on shared rails from affecting already-active domains
Monitoring and Protection:
- Brownout Detection: voltage monitors on critical rails trigger interrupt or reset when supply drops below programmable threshold — response latency must be < 1 μs to prevent data corruption
- Overcurrent Protection: current sensors on regulator outputs detect shorts or excessive load — foldback current limiting reduces output voltage proportionally to prevent thermal damage
- Temperature Monitoring: on-die thermal sensors (BJT-based or ring-oscillator-based) feed PMU for thermal throttling decisions — DVFS reduces voltage/frequency when junction temperature exceeds threshold
- Power Good Signals: each regulator generates a power-good flag when output settles within specification — sequencing logic gates subsequent domain power-up on upstream power-good assertion
PMU integration represents the critical infrastructure layer that enables aggressive multi-domain power management in modern SoCs — without reliable voltage generation, sequencing, and monitoring, advanced power-saving techniques like DVFS, power gating, and retention would be impossible to implement safely.